Add KubeletInUserNamespace feature gate

Enables support for running kubelet in a user namespace.
The user namespace has to be created before running kubelet.
All the node components such as CRI need to be running in the same user namespace.

- Tracking issue: kubernetes/enhancements issue 2033
- KEP: https://github.com/kubernetes/enhancements/tree/master/keps/sig-node/2033-kubelet-in-userns-aka-rootless
- Implementation: kubernetes/kubernetes PR 92863

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

content/en/docs/reference/command-line-tools-reference/feature-gates.md

@@ -138,6 +138,7 @@ different Kubernetes components.
 | `LocalStorageCapacityIsolationFSQuotaMonitoring` | `false` | Alpha | 1.15 | |
 | `LogarithmicScaleDown` | `false` | Alpha | 1.21 | |
 | `LogarithmicScaleDown` | `true` | Beta | 1.22 | |
+| `KubeletInUserNamespace` | `false` | Alpha | 1.22 | |
 | `KubeletPodResourcesGetAllocatable` | `false` | Alpha | 1.21 | |
 | `MemoryManager` | `false` | Alpha | 1.21 | 1.21 |
 | `MemoryManager` | `true` | Beta | 1.22 | |
@@ -785,6 +786,8 @@ Each feature gate is designed for enabling/disabling a specific feature:
   See [setting kubelet parameters via a config file](/docs/tasks/administer-cluster/kubelet-config-file/)
   for more details.
 - `KubeletCredentialProviders`: Enable kubelet exec credential providers for image pull credentials.
+- `KubeletInUserNamespace`: Enables support for running kubelet in a {{<glossary_tooltip text="user namespace" term_id="userns">}}.
+   See [Running Kubernetes Node Components as a Non-root User](/docs/tasks/administer-cluster/kubelet-in-userns/).
 - `KubeletPluginsWatcher`: Enable probe-based plugin watcher utility to enable kubelet
   to discover plugins such as [CSI volume drivers](/docs/concepts/storage/volumes/#csi).
 - `KubeletPodResources`: Enable the kubelet's pod resources gRPC endpoint. See

content/en/docs/reference/glossary/userns.md

@@ -0,0 +1,28 @@
+---
+title: user namespace
+id: userns
+date: 2021-07-13
+full_link: https://man7.org/linux/man-pages/man7/user_namespaces.7.html
+short_description: >
+  A Linux kernel feature to emulate superuser privilege for unprivileged users.
+
+aka:
+tags:
+- security
+---
+
+A kernel feature to emulate root. Used for "rootless containers".
+
+<!--more-->
+
+User namespaces are a Linux kernel feature that allows a non-root user to
+emulate superuser ("root") privileges,
+for example in order to run containers without being a superuser outside the container.
+
+User namespace is effective for mitigating damage of potential container break-out attacks.
+
+In the context of user namespaces, the namespace is a Linux kernel feature, and not a
+{{< glossary_tooltip text="namespace" term_id="namespace" >}} in the Kubernetes sense
+of the term.
+
+<!-- TODO: https://kinvolk.io/blog/2020/12/improving-kubernetes-and-container-security-with-user-namespaces/ -->